M39 Leica Thread Mount

[j-becker]

It's for a Nikkor EL 2.8 50 & 80. I want to mount them into Beseler 45 lens boards (I just bought one for a song on FB since my kids really want to try real film stuff and gave me a reason to dust off my remaining darkroom stuff). Unfortunately I lost the jam nut a few decades ago since my last enlargers (Devere 504 and Dunco 67) did not need a retaining nut.